1. Field of the invention
The present invention relates generally to a circuit for operating a magnetometric sensor, and relates in particular to a voltage step-up circuit for a magnetometric sensor using a SQUID (superconducting quantum interference device), especially an oxide type of SQUID which is formed with a so-called "high-temperature superconductor".
2. Description of Related Art
It is well known that a SQUID is a highly sensitive magnetometric sensor which can measure very weak magnetic fields. If such a SQUID is formed with a high-temperature superconductor of the oxide type, the SQUID can operate at a relatively high temperature -196.degree. C. of liquid nitrogen (N), and accordingly can be handled easily. Therefore, the system handling this type of SQUID is remarkably simplified in contrast to a system using liquid helium (He) at a very low temperature of -269.degree. C., which is difficult to handle. Consequently, the high-temperature type of SQUID is expected to be developed and utilized in various applications, such as medical diagnosis, non-destructive examination, food inspection geological survey, and the like, and many researchers will research and actively develop applications of the SQUID. Thus, the inventors have proposed an invention named "a circuit device for driving a magnetometric sensor," provided with both functions of a SQUID magnetometer and a SQUID characteristic evaluator, in Japanese Patent Application No. 295758/1996, filed on Oct. 17, 1976.
Now, when a SQUID is used in measuring very weak magnetic fields, there is adopted a system with a so-called "FLL" (magnetic flux locked loop), for example, as shown in FIG. 1A, in the prior art. Namely, a SQUID sensor S filled with liquid helium or nitrogen is provided with a SQUID element E and a feedback coil FC, and a driving and processing circuit DP is provided with a bias current source B, a frequency modulation oscillator M, a phase detector PD and a feedback amplifier FA. A desired magnetic field measurement signal V.phi. representing an external magnetic field to the SQUID sensor S is outputted from the amplifier FA. In this case, a small voltage Vd generated by the SQUID element E is stepped up and amplified by a voltage step-up means SA including a first transformer T.sub.1, a second transformer T.sub.2 and an amplifier A. The first transformer T.sub.1, so-called "superconducting transformer," is placed within the SQUID sensor S and operates at a low temperature, for example, at a very low temperature -269.degree. C. of liquid helium (He) or at a relatively high temperature -196.degree. C. of liquid nitrogen (N), while the second transformer T.sub.2 and the amplifier A operates at room temperature.
In FIG. 1A, the SQUID element E responds to an external magnetic field to be measured, and thereby generates the small voltage Vd. This voltage Vd is once stepped up to a predetermined value by the first transformer T.sub.1 in liquid helium or nitrogen within the SQUID sensor S, and then the stepped up voltage is picked up on the exterior of the sensor S. The stepped up voltage is isolated from the following stages and further stepped up by the second transformer T.sub.2 at room temperature. In the prior art, the SQUID voltage Vd is stepped up by the voltage step-up means SA ranged between the interior and the exterior of the SQUID sensor S to be processed thereafter by the phase detector and the following stages thereof.
Thus, there is no risk that an external noise enters the small voltage Vd or that the variation of temperature affects adversely the first transformer, because the first transformer T.sub.1 acting as the first stage of the voltage step-up means SA is within the SQUID sensor S and under a certain constant temperature condition.
However, in this case, the first transformer T.sub.1 must be formed with expensive materials fitted for a very low temperature, and the size of the SQUID sensor S must be large because transformer T.sub.1 is incorporated within the sensor S. Further, as the core material of the first transformer T.sub.1 is cooled to a very low temperature, this degrades its magnetic permeability, and the transformer T.sub.1 cannot achieve desired performance. Furthermore, the use of two transformers T.sub.1 and T.sub.2 naturally requires consideration of matching between these transformers, and the like. Therefore, these problems present great difficult matters that prevent the SQUID from utilization in a magnetometric sensor.
In addition, the above system in the prior art requires many terminals as shown in FIG. 1B. FIG. 1B shows a schematic block diagram of the same magnetometric sensor system as FIG. 1A, and consequently the identical numerals indicating the same elements between FIGS. 1A and 1B are used, but FIG. 1B illustrates, in partial detail, terminals and wirings between a SQUID sensor S and a driving and processing circuit DP.
In FIG. 1B, a bias current source circuit B supplies a bias current signal through bias current output terminals tb.sub.1 and tb.sub.2 to a SQUID element E, the SQUID element E generates a small voltage signal Vd responding to an external magnetic field to be measured, and a first transformer T.sub.1 steps up once the voltage signal Vd to a predetermined value and sends out the stepped up voltage signal as a voltage detection signal through detection signal input terminals td.sub.1 and td.sub.2 to the exterior of the SQUID sensor S. Thus, these four signal terminals tb.sub.1, tb.sub.2, td.sub.1 and td.sub.2 relay signals between the SQUID sensor S and the driving and processing circuit DP.
In addition, such a feedback amplifier sends a feedback signal mixed with a frequency modulation signal from an oscillator M through feedback terminals tf.sub.1 and tf.sub.2 to a feedback coil FC within the SQUID sensor S, and therefore, the circuit DP requires further two feedback terminals tf.sub.1 and tf.sub.2, six terminals in all, for one SQUID sensor.
Thus, in the prior art, it is necessary for a driving and processing circuit to prepare at least four signal terminals for one SQUID element, that is, to adopt a circuit structure of a type to be called as "four terminals manner." These terminals include a first set of two output terminals tb.sub.1 and tb.sub.2 for supplying a bias current signal to the SQUID element, and a second set of two input terminals td.sub.1 and td.sub.2 for picking up a voltage detection from the SQUID element. Further, if feedback terminals tf.sub.1 and tf.sub.2 are included, the circuit requires six signal terminals for one SQUID sensor. Accordingly, in the prior art, there are also disadvantages that a large number of terminals and wirings are required for transmitting and receiving signals between a SQUID sensor and a driving and processing circuit, and correspondingly a large quantity of liquid helium or nitrogen tend is likely to evaporate our of the SQUID sensor.